Regulation of cardiac patterning by the BAF complex subunit Smarcc1a

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PROJECT SUMMARY Cardiac patterning is an essential process that creates distinct regions with unique functions within the embryonic heart. As the heart develops, it transforms from a simple linear heart tube into a patterned structure with atrial and ventricular chambers and the atrioventricular canal (AVC) at their junction. The formation of these morphological distinctions is accompanied by dynamic changes in gene expression patterns: genes that were at first broadly expressed throughout the heart tube become restricted to either the chambers or the AVC. Despite the functional importance of AVC and chamber patterning, we do not fully understand the molecular mechanisms that promote the restriction of specific genes to either region. To address this open question, our laboratory has carried out genetic screens in zebrafish in search of mutations that disrupt the morphology of the AVC or the cardiac chambers. In a previous screen, we identified a mutant with severe defects in AVC and chamber morphogenesis, resulting in a relatively linear heart without a morphologically distinct AVC at the boundary between the atrium and the ventricle. In addition, the mutant heart fails to exhibit the appropriate refinement of gene expression to the AVC. Positional cloning revealed that the causative mutation disrupts the zebrafish smarcc1a gene, encoding a SWI/SNF-type ATP-dependent chromatin remodeling complex subunit homologous to mammalian BAF155. These results suggest a novel model in which chromatin remodeling by Smarcc1a-containing BAF complexes refines the gene expression patterns that promote the distinction between AVC and chamber identities. To test this model, I will first determine when and where Smarcc1a acts to restrict gene expression to the AVC. Specifically, I will examine the extent to which smarcc1a influences regional patterns of gene expression, determine when gene expression defects first emerge in smarcc1a mutants, and utilize transgenic strategies for tissue-specific rescue of smarcc1a mutants. Additionally, I will identify effector genes that act downstream of Smarcc1a to drive cardiac patterning. Using loss-of-function, gain-of-function, and epistasis analysis, I will test whether smarcc1a controls cardiac patterning through its influence on repression of tbx2b expression. I will also use transcriptomic approaches to identify a broader roster of candidate genes that are regulated by smarcc1a, and I will test whether top candidates are key effectors in executing cardiac patterning. Together, these studies are likely to reveal a novel role of smarcc1a in regulating the mechanisms that distinguish the traits of the AVC from the traits of the adjacent cardiac chambers. This role of smarcc1a would represent a previously unappreciated function for BAF chromatin remodeling complexes during cardiac patterning. Thus, this work has the potential to have a broader biomedical impact, as it may enhance our understanding of how mutations in genes encoding components of chromatin remodeling complexes could relate to the origins of congenital heart disease.